Hydrolysis of N2O5 on submicron sulfuric acid aerosols

Article abstract of DOI:10.1021/jp9939625

The hydrolysis of N₂O₅ on the surfaces of aerosol particles and cloud droplets is the dominant removal channel for NO_x at night. Thus, it is important in determining the oxidative capacity of the atmosphere as the concentrations of NO_x strongly affect the rate of tropospheric O₃ formation. NO_x levels in the stratosphere were depleted due to hydrolysis of N₂O₅ on the large quantity of sulfuric acid aerosol generated by volcanic gases. The kinetics of reactive uptake of gaseous N₂O₅ on submicron sulfuric acid aerosol particles was studied using a laminar flow reactor coupled with a differential mobility analyzer to characterize the aerosol. At 26.3-64.5 wt % H₂SO₄ concentration, the uptake coefficient (γ) was 0.033 ± 0.004, independent of acid strength and of relative humidity at 8-80%. For an acid strength of 45 wt %, Γ decreased with increasing temperature at 263-298 K. Temperature dependence values of -115 ± 30 kJ/mole and -25.5 ± 8.4 J/mole were determined for the changes in enthalpy and entropy of the uptake process, respectively. The results were consistent with a previous model of N₂O₅ hydrolysis involving a direct and an acid catalyzed mechanism, with uptake under the experimental conditions limited by mass accommodation.

Full text of DOI:10.1021/jp9939625

3984
J. Phys. Chem. A 2000, 104, 3984-3990
Hydrolysis of N2O5 on Submicron Sulfuric Acid Aerosols
†
Mattias Hallquist, David J. Stewart, Jacob Baker, and R. Anthony Cox*
Centre for Atmospheric Science, Department of Chemistry, UniVersity of Cambridge, Lensfield Road,
Cambridge CB2 1EW, U.K.
ReceiVed: NoVember 8, 1999; In Final Form: February 14, 2000
The kinetics of reactive uptake of gaseous N
investigated using a laminar flow reactor coupled with a differential mobility analyzer (DMA) to characterize
the aerosol. The particles were generated by homogeneous nucleation of SO /H O mixtures. In the H SO
2 5
O on submicron sulfuric acid aerosol particles has been
3
2
2
4
concentration range 26.3-64.5 wt % the uptake coefficient was γ ) 0.033 ( 0.004, independent of acid
strength. For an acid strength of 45 wt % γ was found to decrease with increasing temperature over the range
2
63-298 K. From this, temperature dependence values of -115 ( 30 kJ/mol and -25.5 ( 8.4 J/K mol were
determined for the changes in enthalpy and entropy of the uptake process, respectively. The results are consistent
with a previous model of N hydrolysis involving both a direct and an acid catalyzed mechanism, with
uptake under the experimental conditions limited by mass accommodation.
2 5
O
Introduction
260 K), but there is less information for tropospheric conditions.
There is also some disagreement as to the effect of relative
humidity, i.e., the acid strength of the aerosol, on the uptake
coefficient. Hu and Abbatt present results in which the uptake
It is now well-known that hydrolysis of N2O5 on the surfaces
of aerosol particles and cloud droplets is the dominant removal
channel for nitrogen oxides at night. It is therefore important
in determining the oxidative capacity of the atmosphere as the
concentrations of NOx strongly influence the rate of tropospheric
ozone formation.
4
coefficient shows a tendency to decrease with increasing
humidity (decreasing acid strength) over the range 8.5-90%,
5
whereas the model developed by Robinson et al., based on
literature data and their studies of uptake onto large droplets
and bulk surfaces, predicts only a very slight increase of the
rate of uptake as the acid strength is raised. The motivation of
this study was therefore 2-fold. First to determine the rate of
uptake of N2O5 on sulfuric acid aerosol under tropospheric
conditions and second to compare our data to that of Hu and
N O (g) + H O(surface) f 2HNO (aq)
(R1)
2
5
2
3
Reaction R1 converts reactive nitrogen oxide species (NOx)
into the longer-lived reservoir species HNO3, which is then
removed from the atmosphere via wet or dry deposition. It has
4
5
1
Abbatt and Robinson et al. to confirm whether or not the
uptake is dependent on the acid strength of the aerosol.
In this study the uptake of N2O5 on sulfuric acid aerosols
was measured over a wide range of acid concentrations (26.3-
been shown by Dentener and Crutzen that the inclusion of this
reaction in a global tropospheric model, assuming an uptake
coefficient of 0.1 on ammonium hydrogen sulfate aerosol,
decreases the NOx and O3 levels by 45% and 10% with respect
to values obtained by considering only gas-phase chemistry in
the summertime at mid-latitudes (45° N, 500 hPa, mid-June).
Even larger relative changes are predicted for higher latitudes
and in the planetary boundary layer. This reaction is also
important in the stratosphere, a fact that was illustrated after
the eruption of Mount Pinatubo. The NOx levels in the
stratosphere were significantly depleted due to hydrolysis of
N2O5 on the large quantity of sulfuric acid aerosol generated
6
4.5 wt %) at room temperature corresponding to a relative
humidity (RH) range of 8-80%. The temperature dependence
of the uptake was determined over the range 263-298 K for
an acid concentration of 45 wt %. Since these measurements
have been taken over a wide range of relative humidity, they
allow a reliable assessment of the dependence of the reactive
uptake coefficient on the water content of the aerosol. The
resulting relative humidity and temperature dependences are
discussed. Comparisons of these results are made with preexist-
ing models of the uptake of gaseous species on aerosol particles.
The use of submicron aerosol particles for these studies has
two advantages. The first is to minimize the effects of gas-
phase diffusion to the acid surface on the uptake rates, which
can be a problem when bulk surfaces are used. Second the
sulfuric acid present in the atmosphere is contained in small
particles such as these, so the experiment is a better representa-
tion of what is actually occurring in the atmosphere.
2
by the volcanic gases. The reactive uptake coefficient of N2O5
is dependent on the characteristics of the aerosol such as the
particle surface to gas volume ratio, the chemical composition
of the aerosol, and the corresponding reactivity of N2O5 on the
surface.3
A major source of uncertainty in model estimates of the
impact of heterogeneous reactions is the lack of certainty in
the values used for the uptake coefficients. In the recent past,
the hydrolysis of N2O5 with H2SO4/H2O aerosol particles has
been studied in detail under stratospheric conditions, i.e.
concentrated H2SO4 (50-80 wt %) at low temperature (185-
Experimental Section
Uptake coefficients of N2O5 on sulfuric acid aerosol particles
were determined using a recently built aerosol flow tube system
described by Baker et al. The system described in that paper
*
To whom correspondence should be addressed. E-mail: rac26@cam.ac.uk.
Present address: School of Sciences, University of Sunderland, Benedict
†
6
Building, St. George’s Way Sunderland SR2 7BW, U.K.
1
0.1021/jp9939625 CCC: $19.00 © 2000 American Chemical Society
Published on Web 04/27/2000

N2O5 Uptake on Submicron Sulfuric Acid Aerosols
J. Phys. Chem. A, Vol. 104, No. 17, 2000 3985
constant to within (7% over the time of a kinetic run (ca. 1/2
-
1 h). The aerosol surface area typically varied by less than
0% over the same period.
1
Aerosol Characterization. The aerosol was characterized
using a differential mobility analyzer (DMA) (Hauke EMS
VIE-08) with a Faraday cup electrometer serving as the
detector. The DMA output is a number density per size channel
with software corrections made for the fraction of particles
charged, the slip factor, and the effects of multiple charging.
The working range of the DMA was set up to measure particle
radii of 0.02-0.5 µm, with measurements being made at 35
selected size channels over 5 min. The characterization of the
aerosol using the DMA was compared to the method using UV
spectroscopy and particle counting as were used in a previous
6
study. The difference in measured surface area was less than
5
% between these methods. However, the DMA is easier to
use and has a lower detection limit, and hence the DMA was
used to characterize the aerosol in all kinetic measurements.
Since most of the experiments were done at high relative
humidity, the sheath air flow in the DMA was humidified to
match the relative humidity of the aerosol flow to prevent the
aerosol particle sizes from changing during sampling.
Figure 1. Schematic diagram of the flow tube system. Shaded areas
are temperature controlled.
has been modified, and a schematic diagram of the revised
system is shown in Figure 1. Four main subsystems connected
to the flow tube are aerosol generation, aerosol characterization,
N2O5 delivery, and the detection system. These will be described
in turn.
For the low-temperature data, corrections were made to the
measured aerosol surface area to take into account the difference
in temperature and, hence, relative humidity in the flow tube
and the DMA. Thermodynamic data derived from Perry and
7
Green were used.
Aerosol Generation. The sulfuric acid aerosol generator is
based on the homogeneous nucleation of SO3 and water. Sulfur
trioxide was entrained into a filtered nitrogen stream (MG Gases)
by passing the nitrogen over fuming sulfuric acid (BDH, 30%
SO3). The sulfur trioxide laden gas stream was then injected
axially into a nucleation chamber where it came into contact
with a sheath flow of humidified nitrogen. Sulfuric acid particles
were produced upon mixing and formed a column of particles
down the chamber. On exiting the nucleation chamber, the
aerosol entered a 250 mL round-bottomed flask, where it was
diluted with a further flow of humidified nitrogen and mixed
with a Teflon coated magnetic stirrer to ensure the aerosol
particles were homogeneously distributed in the gas stream. The
product from this system is a stable aerosol under a specifically
chosen condition, i.e., RH and temperature. The relative
humidities in the aerosol generation system were monitored
using Humitter 50Y probes (Vaisala) which have a precision
of ca. (2%. On exiting the aerosol generation system, the
aerosol flow is sent to a switching valve which diverts it either
to the reaction cell or to a vent. If the aerosol is directed to the
vent, the flow is replaced by an equal bypass flow of humidified
nitrogen. Before entering the cell the flow is passed through
two Pyrex glass conditioners. The first (3 cm i.d., 55 cm length)
containing a bulk solution of sulfuric acid at the appropriate
concentration, which is used to fine-tune the concentration of
the aerosol. The relative humidity is controlled by the equilib-
rium vapor pressure of water above the sulfuric acid solution.
The next conditioner (3.5 cm i.d., 60 cm length) was used to
precool the aerosol particles when working at lower tempera-
tures. After the two conditioning flow tubes, the aerosol was
N O Delivery System. N O was synthesized prior to use
2
5
2
5
by mixing NO and excess O , in a flow system. The N O
5
3
2
produced was trapped and stored at 195 K. In the delivery
system the solid N O was held in a cold trap at 240 K, and a
2
5
slow flow of dried nitrogen gas was used to entrain the N O .
2
5
This flow could either be directed to the flow system or to the
vent and was prediluted with additional nitrogen to give a total
3
-1
flow of about 50 cm (STP) min of N O . This mixture was
2
5
delivered to the aerosol flow tube by means of a sliding stainless
steel injector. The N O stream was passed through the inside
2
5
1
of the injector using a / in. Teflon tube. The end of the injector
8
is fitted with a Teflon “pepper pot” to enhance the mixing of
this stream with the aerosol. To minimize the effects of wall
loss, the interior surface of the flow reactor was coated with
halocarbon wax.
N O Detection. N O was detected at the exit of the flow
2
5
2
5
8
3
-1
tube by titration with NO. A flow of 30 cm (STP) min of a
mixture of 30 ppm NO in nitrogen (Messer) was used, giving
an NO concentration of approximately 700 ppb. NO was added
contra flow to the main gas stream at the end of the flow tube.
On exiting the flow tube, the sample gas stream passed through
a heated glass tube, where the N O thermally decomposes:
2
5
N O f NO + NO
3
(R2)
2
5
2
The NO produced is then titrated with the excess NO to produce
3
NO2:
NO + NO f 2 NO
(R3)
3
2
3
-1
divided into two flows. A flow of 2800 cm (STP) min was
directed to the differential mobility analyzer (DMA) and a flow
The amount of N2O5 at the exit of the flow tube can therefore
be obtained by measuring the change in concentration of NO
when the N2O5 stream is switched on and off. NO was detected
by means of a commercial nitrogen oxides monitor (Monitor
Labs model 8440). This detector has a sampling flow rate of
3
-1
of 1000 cm (STP) min directed into the flow reactor (3.2 cm
i.d. and 80 cm length). In this way the aerosol characteristics
could be monitored during the course of an experiment. From
the aerosol characterization it was found that the aerosol
generator takes approximately 1 h to stabilize after setting the
flows. After this time the particle number density remained
3
-1
450 cm (STP) min and can measure NO concentrations up
to 1 ppm. The detection limit of the instrument was 5 ppb (1.25
1
1
-3
× 10 cm ) with a S/N ) 2. The change in NO was shown to

3986 J. Phys. Chem. A, Vol. 104, No. 17, 2000
Hallquist et al.
TABLE 1: Aerosol Characteristics and Concentrations Used
in the Experiments
no. (cm-3)
(1.5-4) × 10
(1.8-15) × 10
5
2
3
-4
area (cm /cm )
3
3
-10
vol (cm /cm )
peak radius (nm)
RH (%)
(6.5-100) × 10
120-220
8-80
[
[
H
N
2
SO
4
] (wt %)
26.3-64.5
3
12
2
5
O ] (molecules/cm )
(2.5-12.5) × 10
be a linear function of the flow rate through the N2O5 trap. To
prevent damage to the analyzer, a filter removing particulate
matter was fitted in the sample line. Nitric acid produced in
the reaction was removed by using nylon tubing, which adsorbs
HNO3 between the exit of the flow tube and the NO detector.
Both the filter and the nylon tubing are inert to NO, so the
concentrations of NO were not affected. NO2 present in the gas
stream did not interfere with the NO measurements.
Figure 2. An example of a ln [N
2
O
5
] vs contact time plot for the
uptake of N O on sulfuric acid aerosol: filled and open squares, wall
2
5
loss before and after the aerosol experiment; filled circles, loss in the
presence of aerosol particles. Data shown for 40% RH at 298 K. Aerosol
surface area per volume of gas ) 0.085 m . Errors shown are estimated
from the noise in the measured signal.
Procedure. The flows entering the reaction chamber from
the aerosol generator were set so that the total flow was 3.8
-1
3
-1
sLpm. The DMA sampled 2800 cm (STP) min leaving a flow
3
-1
of 1000 cm (STP) min of the aerosol flow to enter the reaction
chamber. Under these conditions the flow in the tube is laminar
and well-developed at 99% of the final velocities after about 2
cm. The time for diffusional mixing is estimated to be about 5
s, assuming a diffusion coefficient of 0.1 cm /s for N2O5 in air
TABLE 2: Reactive Uptake Coefficients Measured at Each
Relative Humidity at 298 Ka
RH
(%)
[H
(wt %)
2
SO
4
]
no. of
determinations
9
γav
2
8
10
64.5 ( 1.7
64.5 ( 1.7
57.0 ( 1.1
52.9 ( 1.1
47.8 ( 1.0
43.1 ( 0.9
33.2 ( 0.9
26.3 ( 1.5
26.3-64.5
0.035 ( 0.007
0.039 ( 0.007
0.041 ( 0.007
0.030 ( 0.004
0.028 ( 0.008
0.040 ( 0.006
0.039 ( 0.005
0.028 ( 0.013
0.033 ( 0.004
4
2
4
13
5
4
5
6
43
4
as calculated by Hu and Abbatt. No data were taken in this
22
30
40
50
70
mixing region, which corresponded to the first 10 cm after the
N2O5 inlet.
The characteristics of the aerosol and the concentrations used
in the experiments are summarized in Table 1. The experimental
procedure was to determine a first-order loss of N2O5 by
changing the position of the sliding injector to vary the contact
time (20-45 s) between the aerosol and the N2O5 and
monitoring the change in N2O5 concentration. The aerosol flow
was switched out of the cell before and after each kinetic run,
and an experiment carried out in the absence of aerosol in order
to discriminate between N2O5 wall loss and N2O5 loss to the
aerosol. The rate constant for reaction with and without aerosol
present, obtained from the slope of a plot of ln[N2O5] against
contact time, was then corrected for non-plug-flow conditions.
First the observed wall loss rate constant was corrected for the
effect of diffusion to the walls on the concentration of N2O5 by
solution of the diffusion equation with first-order kinetics as
described by Brown.10 Appropriate values of the flow velocity,
diffusion coefficient, radius of the flow tube, and the measured
wall loss were used. The correction of the observed decay rate
coefficient in the presence of aerosol for the effects of wall loss
under non-plug-flow conditions was then made by an iterative
80
8
-80
a
Stated errors on the mean γ at 95% confidence level. Errors on
H SO4 wt % result from a (2% error in the relative humidity
measurement.
2
complete form derived by Fuchs and Sutugin.11 The radius used
in the calculation is at the peak of the measured surface area
distribution. However, since the contribution of the diffusion
factor (a/DN O ) is small, the reactive uptake coefficient did not
2
5
significantly change when a was varied. For the low-temperature
work the diffusion coefficients were calculated using the formula
1
2
of Lovejoy and Hanson (eq 2).
1
.94
6
3.92 (T/K)
P/Torr) 293
D
)
5
(2)
N O
(
)
2
(
Result and Discussion
10
procedure also described in the paper by Brown. The diffusion
equation was solved for appropriate values of the corrected wall
loss for the same conditions, the diffusion coefficient, radius
of the flow tube, and flow velocity. The uncorrected rate
coefficient was multiplied by 1.2 and entered as a “first guess”
in the iteration process. The reactive uptake coefficient, γ, was
then calculated from this final corrected rate coefficient, k, using
eq 1, where A is the surface area of the aerosol per unit volume
Kinetics of N2O5 Uptake. A typical plot showing the result
of a kinetic run is shown in Figure 2. From this plot it is apparent
that the rate of N2O5 loss is significantly greater in the presence
of aerosol than in its absence. Second, the decay of N2O5 is
exponential as expected for a first-order loss reaction. The first-
order decay constant was then corrected according to the
procedure above. From the corrected decay rate coefficients the
uptake coefficients, γ, were calculated using eq 1 for each set
of experimental conditions. Using this procedure, γ for the
reaction of N2O5 was determined over a range of relative
humidities from 8 to 80% or H2SO4 concentrations of 26.3-
a
4
〈c〉γ
k ) A/
+
(1)
(
D
)
N O
2
5
6
4.5 wt %. The results are shown in Table 2 and plotted in
-1
of carrier gas (cm ), a is the radius of the particles (cm), DN O
is the diffusion coefficient for N2O5 in nitrogen (cm s ), and
c〉 is the average speed of the gas molecules (cm s ). The
Figure 3. The data points shown have been obtained from a
number of determinations at each relative humidity as shown
in the table. The results are quite reproducible, as shown by
the small error which is the standard deviation on the mean of
the stated number of experiments at the 95% confidence level.
2
5
2
-1
-
1
〈
calculated γ obtained using eq 1 was found not to be
significantly different from that calculated using the more

N2O5 Uptake on Submicron Sulfuric Acid Aerosols
J. Phys. Chem. A, Vol. 104, No. 17, 2000 3987
Figure 3. Uptake coefficient of N
2 5
O on aqueous sulfuric acid solutions
2 5
Figure 5. Uptake coefficient for N O on sulfuric acid aqueous
as a function of relative humidity at 298 K: filled circles, this work;
solutions as a function of temperature at 40% RH: filled circles, this
4
12
crosses, Hu and Abbatt; triangles, Lovejoy and Hanson; open circles,
5
work; crosses, Robinson et al; filled square, Hanson and Ravishan-
1
3
16
Fried et al.; squares, Mozurkewich and Calvert; diamond, George
18
kara. Errors shown for this work are at the 95% confidence level of
2
5
et al. Errors shown for this work are at the 95% confidence level of
the average of a number of determinations.
the average of a number of determinations.
TABLE 3: Reactive Uptake Coefficients Measured at
a
Different Temperatures at 40% Relative Humidity
temp [H
2
SO
4
]
γ
wall
no. of
determinations
6
(K)
(wt %)
γ
av
(×10 )
298
288
278
268
263
47.8
47.7
44.3
42.2
42.1
0.028 ( 0.008
0.036 ( 0.004
2.78 ( 1.02
1.45 ( 0.08
5
4
3
2
3
0.049 ( 0.009 10.40 ( 2.13
0.091 ( 0.019
0.092 ( 0.013
6.64 ( 1.34
7.56 ( 1.53
a
Stated errors are at 95% confidence level.
which point no further increase is apparent, although the
reproducibility in this region is poor. This observed behavior
could be due to the build up of a surface layer of water on the
interior wall of the flow tube, which increases in extent with
relative humidity. The magnitude of γw at low humidity is small,
as expected for a nonpolar surface, i.e. the halocarbon wax
coating. The end of the exponential increase at ∼70% humidity
could be due to the fact that above this point the liquid film is
extensive enough that the amount of water present to absorb
the N2O5 on the wall is no longer the factor limiting the rate of
wall loss. The measured γw is significantly smaller at low RH
Figure 4. Observed wall loss (plotted as γwall) as a function of relative
humidity. The shown line is a quadratic polynomial fit to the data.
Errors shown are at the 95% confidence level of the average of a number
of determinations.
Room temperature literature values are shown in Figure 3
for comparison. The uptake coefficients agree well with those
recently reported by Lovejoy and Hanson12 and Fried et al.,13
both of which were measured on small aerosol particles. Our
data are also consistent with the results presented by Van Doren
-
6
than the value of γω > 6 × 10 reported by Hanson and
1
7
Lovejoy for uptake on the glass surface of their flow tube at
low relative humidity. At moderate humidities the wall loss
reported in their work is comparable with our measurements.
Similar behavior has been observed by George et al.25 in their
study of the uptake of N2O5 on NaCl solutions.
1
4
et al. for the uptake on large (60 µm) droplets of water and
15
strong acid and the work of Kirchner et al. using a liquid water
4
jet. The uptake coefficients presented by Hu and Abbatt agree
numerically quite well with our data, but the apparent influence
of relative humidity on the uptake coefficient hinted in their
work was not found in our measurements. The results of
Mozurkewich and Calvert16 do not agree with the other data,
although it should be noted that this was the first attempt to
measure the uptake coefficient of N2O5 on sulfuric acid aerosol
and the errors were very large. Their results were in fact
superseded by new measurements from the same laboratory
published by Fried et al.13
Temperature Dependence of N2O5 Uptake. The reactive
uptake coefficients were measured over the temperature range
2
63-298 K at a fixed relative humidity of 40%, corresponding
to an acid strength of about 45 wt %. From the results shown
in Figure 5 and Table 3 it is apparent that the uptake coefficient
shows a negative temperature dependence. These data are
consistent with the low-temperature data presented by Robinson
5
et al. for uptake on large droplets, and the work of Hanson
The observed wall loss, kw, at 298 K was found to increase
with increasing relative humidity. Values of the uptake coef-
ficient on the wall, γw, are shown in Figure 4. These data were
1
8
and Ravishankara, which are also plotted in Figure 5. The
temperature dependence of the wall loss, presented in Table 3,
is less pronounced and less reproducible and depends on the
history of the flow tube wall.
10
calculated from the measured wall loss corrected for diffusion,
kw, using eq 3, where r is the radius of the flow tube and ω is
the mean molecular speed.
Mechanism of N2O5 Hydrolysis. The overall uptake of a
reactive gas into liquid droplets that are small enough to avoid
gas diffusion limitations can be expressed in terms of the mass
accommodation coefficient R, and the liquid-phase reactivity,
γ ) 2k r/ω
(3)
w
w
19
The plot shows the uptake on the wall of the flow tube
increases with relative humidity up to approximately 70% at
γreacn, using the resistance equation (e.g. see Schwartz and
2
0
Davidovits et al. )

3988 J. Phys. Chem. A, Vol. 104, No. 17, 2000
Hallquist et al.
1
γ
1
R
1
)
+
(4)
(5)
γreacn + γsoln
where
1/2
γreacn ) (4H*RT/ cj )[Dekreacn
]
Here H* is the effective Henry’s coefficient for trace gas
solubility, De the liquid-phase diffusion coefficient, cj the
molecular velocity, and kreacn the liquid-phase reaction rate
coefficient. In our analysis we have assumed that the solubility
limitation to the uptake rate, γsol., can be ignored. If the liquid-
phase reaction rate is fast, the uptake rate is controlled by
accommodation at the surface, i.e., reaction R4. On the other
hand if the reaction rate is slow, the liquid-phase reaction rate
controls reactive uptake.
Figure 6. Plot of ln[γ/(1 - γ)] vs 1/T. The gradient of this plot gives
-
∆H/R and the intercept ∆S/R: filled circles, our data at ca. 45 wt %
4,5,13,14,17
acid; squares, literature data for ca. 70 wt % acid;
diamonds,
2
5
data for uptake of N on water reported by George et al. Errors
2 5
O
N O (g) H N O (aq)
(R4)
2
5
2
5
shown for this work are at the 95% confidence level of the average of
a number of determinations.
Consideration of the temperature dependence of γ offers a
way of distinguishing which controlling processes operate at
the liquid interface. The process of accommodation at the liquid
interface has been described in terms of a thermodynamic model
TABLE 4: Enthalpies, Entropies and Critical Cluster Sizes
H SO4
(wt %)
2
∆Hobs
(kJ/mol)
∆Sobs
N
N
((J/K)/mol) (from ∆Hobs
)
(from ∆Sobs
)
20
for uptake (Davidovits et al. and reviewed by Nathanson et
7
45
0
0
-18.9 ( 7.6
-25.0 ( 8.4
-103 ( 39
-115 ( 30
1.88
2.09
2.39
1.82
2.14
2.54
2
1
al. ). In this theory the incoming molecule has to pass through
a sharp but finite transition region several molecular diameters
in thickness within which the density increases from the gas to
the liquid phase. This model uses the concept that this narrow
region is a dense gaslike state in which nucleation around the
incoming molecule is constantly occurring. The equilibrium
density of clusters containing N molecules is proportional to
exp (-∆GN/RT) where ∆GN is the molar Gibbs energy for the
formation of a cluster with N molecules. ∆GN increases with
cluster size, reaches a maximum, and then decreases. This
implies that clusters smaller than a certain critical size evaporate
whereas clusters larger than this serve as condensation centers,
grow in size and become part of the bulk liquid. Davidovits et
-33.5 ( 18.5 -156 ( 78
a
N ) average number of molecules in a critical cluster.
2
/3
kJ mol-1 (7)
J K-1 mol
∆
H ) -41.8(N - 1) + 31.48(N - 1)
2
/3
-1
∆
S ) -123.31(N - 1) + 38.5(N - 1)
(8)
For single clusters it is obvious that the critical number must
be an integer, but the observed noninteger values can be
interpreted as representing the average number of molecules in
critical clusters across the interfacial region. The number of
molecules required to form a critical cluster with N2O5 is small,
indicating that critical clusters form relatively easily. However,
the distinct decrease in critical cluster size as the acid strength
increases shows that critical clusters form more easily in strongly
acidic solutions.
2
0
al. suggest that gas uptake proceeds via the growth of critical
clusters; i.e. if the incoming gas molecule is a part of a critical
cluster, it becomes adsorbed into the solution. The temperature
dependence of R, or γ if the uptake is accommodation
controlled, is expressed in terms of the free energy for
q
13
accommodation, ∆G obs, i.e. the Gibbs energy of the transition
At lower temperatures (<260 K) both Fried et al. and
5
state between the gas phase and the aqueous phase solvation.
Robinson et al. have shown that the temperature dependence
of γ declines, and for the lowest temperatures and highest acid
strengths there is a decrease in γ as temperature falls. Clearly
q
obs
5
-∆G
R
- R
)
exp
(6)
(
)
accommodation is not rate determining under these conditions,
and the liquid-phase chemistry is playing a role. This decline
in γ is not very sensitive to the sulfuric acid concentration. This
1
RT
q
q
q
Since ∆G ) ∆H - T∆S the values for enthalpy and entropy
for accommodation of N2O5, ∆Hobs and ∆Sobs, can be obtained
by plotting ln [γ/(1 - γ)] vs 1/T. Figure 6 shows a plot of the
measured data, at 45 and 70 wt % H2SO4, as well as the data
for uptake on pure water reported by George et al.25 according
to eq 6. The values for ∆Hobs and ∆Sobs obtained from the
gradient and the intercept are shown in Table 4. These results
fit very well with the data for a wide range of compounds
presented by Davidovits et al.,20 which show a linear correlation
between ∆Hobs and ∆Sobs for accommodation into liquids. Thus
the observed temperature dependence is consistent with mass
accommodation control for reactive uptake of N2O5. From the
theory presented by Davidovits et al.,20 the average number of
molecules, N, required to form a critical cluster large enough
to ensure uptake of the N2O5 can be calculated using the
obtained values of ∆Hobs and ∆Sobs, respectively (eqs 7 and
1
6
was explained by Mozurkewich and Calvert and later Fried
1
3
et al.. by assuming that the limiting step in the mechanism is
rapid autoionization of N2O5 at the surface,
-
+
N O (aq) H NO + NO2
(R5)
2
5
3
which does not involve H2O. Alternatively, Robinson et al.5
suggested that two reaction channels were operating, a direct
reaction with water (R6) and an additional acid-catalyzed
reaction (R7) which serves to maintain the overall reaction rate
as the water activity is reduced in strong acid solution.
+
-
N
O
(aq) + H
O f 2H + 2NO
(R6)
(R7)
2
5
2
3
+
+
20
N O (aq) + H f HNO + NO
2 5 3 2
8
); these cluster sizes are also shown in Table 4.

N2O5 Uptake on Submicron Sulfuric Acid Aerosols
J. Phys. Chem. A, Vol. 104, No. 17, 2000 3989
A phenomenological model based on this mechanism gave a
good description of measured rates on both aerosols and bulk
solutions over a fairly wide range of acid concentrations and
temperatures, using a single consistent set of parameters for
coefficient as the water activity falls in strong acid solution. If
the compensating effect of reactions R6 and R7 is the explana-
tion of the observed behavior at low temperatures, it must be
concluded that the importance of the autoionization of N2O5,
reaction R3, is diminished at low temperatures.
Atmospheric Implications. The work presented in this paper
has provided new data for the uptake and hydrolysis of N2O5
on sulfuric acid aerosols. Reliable data are now available for
the reactive uptake coefficient of this reaction over the range
of temperatures, acid strength, and relative humidity expected
in the troposphere and the stratosphere.
*
accommodation (R) and liquid-phase reaction (H , De, kreacn).
In both the mechanisms of Fried et al.13 and Robinson et al.,
5
HNO3 is eventually produced via reaction R8 and/or R9.
NO2+ + H O f H + HNO
+
(R8)
(R9)
2
3
NO3- + H f HNO3
+
Our results show that for modeling the global troposphere
(T > 260 K) a value of γ(N O + H O) between 0.03 and 0.1
+
The two mechanisms both involve formation of NO2 , as an
intermediate species, but according to the model of Robinson
2
5
2
should be used depending on temperature. Dentener and
5
+
1
et al. it is only produced at high H activity, i.e. in strong acid.
Although this extensive model has been tested over a range of
conditions for acid particles, it is apparently inconsistent with
Crutzen assumed a value of 0.1 for the uptake coefficient of
N O on ammonium hydrogen sulfate aerosol in their tropo-
2
5
spheric model. However, from a sensitivity study they showed
that this reaction still has an extremely important effect on the
oxidizing capacity of the troposphere even with a value as low
as γ ) 0.01. The effect of including heterogeneous chemistry
in their model with γ ) 0.1 gave decreases in NO , O , and
+
22
the formation of NO2 proposed by Behnke et al. in the
-
reaction of N2O5 with Cl in neutral sodium chloride aerosol
particles. They observed ClNO2 as the main product from the
N2O5 reaction and suggested that it was due to the reaction
x
3
-
+
between Cl and NO2 in the aqueous phase. This mechanism
OH by 49, 9, and 9%, respectively, compared to a base case
with no heterogeneous chemistry. Using the lower reactivity of
γ ) 0.01, the decreases in NO , O , and OH were still
2
3
was later supported by the results of Schweitzer et al. for the
reaction between N2O5 and aqueous sodium halide droplets at
temperatures down to 262 K. However, they observed only a
weak temperature dependence of the measured uptake coef-
ficients on dilute salt droplets, in contrast to our measurements
on acid aerosols. They concluded that the uptake of N2O5 on
dilute NaCl, NaBr, and NaI solutions are not accommodation
limited and that liquid-phase processes control the uptake rate.
x
3
significant, i.e. 40, 4, and 3%.
Conclusion
N2O5 reacts with water in sulfuric acid aerosol particles to
form HNO3. At 298 K the reaction probability is 0.033 ( 0.004
and is independent of relative humidity in the range 8-80%.
The lack of relative humidity dependence of the uptake
coefficient is consistent with possible mechanisms suggested
in the literature.
3
Wahner et al. measured the rate of uptake of N2O5 on sodium
nitrate particles and found that the uptake coefficient decreased
significantly with increased concentration of NO3 in the
aerosol. This inhibiting effect was attributed to the reaction
-
The reactive uptake coefficient for N2O5 on submicron
sulfuric acid aerosol shows a negative temperature dependence
in the range 263-298 K, which is consistent with the higher
values of γ reported earlier for lower temperatures. The
temperature dependence data fit well with the thermodynamic
+
-
between NO2 and NO3 , reforming N2O5 in competition with
+
the hydrolysis of NO2 .
NO3- + NO2 h N O (aq)
+
(R-5)
2
5
2
0
21
model presented by Davidovits et al. and Nathanson et al.,
If reaction R5 is an important step in the hydrolysis
mechanism, the reverse reaction R-5 may affect the uptake
coefficients measured in the present study if nitric acid (as
reproducing the correlation between observed ∆H and ∆S for
the uptake process.
This reaction has an important effect on the oxidizing capacity
of the troposphere, as suggested by the model of Dentener and
-
NO3 ) builds up in the particles over the course of an
experiment. To determine if this was the case, the maximum
concentrations of nitric acid that could be present in the aerosol
resulting from N2O5 hydrolysis was compared with the measured
uptake coefficient. No correlation was found between nitric acid
concentration and the rate of uptake, which implies either that
inhibition does not occur or that the nitric acid formed quickly
degasses from the aerosol particles as the aerosol has a very
1
.
Crutzen When the reaction is included in their mathematical
model of the atmosphere, it leads to a significant decrease in
the tropospheric concentrations of NOx, ozone, and OH and
gives better agreement between the model and measurements
of the concentrations of these species.
Acknowledgment. We gratefully acknowledge the loan of
equipment from Dr. David Ames and Dr. Kevin Clemitshaw.
This work was supported by the EU environment project
2
4
low pH. This is in agreement with the results of Hanson who
has shown that the uptake coefficient for N2O5 at low temper-
atures (200-230 K) only depends slightly on the HNO3
concentration, i.e. a change of around 20% on going from 0 to
“HECONOS” (Contract No. ENV4-C797-0407) and by the U.K.
NERC (Contract No. GST/02/1884). D.J.S. thanks the U.K.
NERC for the award of a studentship.
1
5 wt % HNO3. In addition, the solubility of HNO3 in acid
solutions at higher temperatures is limited.
From the above discussion we conclude that all three initial
reactions R5-R7 of N2O5(aq) are plausible and that heteroge-
neous dissociation is dominant at room temperature in ac-
References and Notes
(
1) Dentener, F. J.; Crutzen, P. J. J. Geophys. Res. 1993, 98, 7149.
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22
23
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(
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(
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1
(
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(
(